This invention relates generally to an image forming apparatus, and more particularly to the exposure mechanism for forming the image on a photosensitive medium.
Generally, printers and copiers use an electrophotographic or silver halide photographic process in which an image is formed on a photosensitive medium. Both the electrophotographic and silver halide photographic processes, however, suffer from several drawbacks.
Copiers and printers in which the electrophotographic process is used generally produce copies having poor tonal quality especially for intermediate tones. Such poor tonal quality is due, in part, to the photosensitive medium being repeatedly exposed to toner transfer and fixing for the yellow, magenta, and cyan colors. The repeated exposure, transfer and fixing of these three color toners can result in an undesirable offset of the image, that is, multiple colored images formed on the transfer medium (e.g. sheet of paper).
To prevent offset, printers and copiers using the electrophotographic process attempt to provide a high degree of precision in the paper feed path of the transfer medium. Printers and copiers with such precision are both large in size and costly to manufacture. The tendency of the transfer medium to become displaced during its path of travel within the printer or copier resulting in offset of the image is difficult to eliminate regardless of the size of the printer or copier.
Materials involved in silver halide photography are not easily handled due to the wet developing process employed. Silver halide photographic processing also generates an offensive odor and requires disposal of the waste solution. The need to use expensive silver halide paper results in an undesirable high operating cost. Silver halide photographic processing also involves a number of complicated steps which increase production costs and require the use of equipment of large size and weight.
Another type of printer or copier (hereinafter referred to as an image forming apparatus) includes a photosensitive medium on which a latent image is formed. The photosensitive medium has a plurality of microcapsules uniformly distributed over a surface of a substrate. Each microcapsule has a diameter of several micrometers and contains dye precursors of either red, green or blue and photosetting substances. The photosetting substance for each microcapsule reacts in response to exposure to the wavelength of light corresponding to the same color as its dye precursor. After exposure to the multicolored light of the image, the microcapsules may remain unaffected by the light or may harden or soften more or less in accordance with the wavelength(s) of the light. The microcapsules not sufficiently hardened are broken partially or completely by pressure exerted by a pair of pressure rollers. The dye precursor from each broken microcapsule escapes and reacts with a developer coated on the surface of the transfer medium to form a color image on the latter.
The colored image produced on the transfer medium has a high resolution of about 100 dots/millimeters. Individual sheets of paper are typically used as the transfer medium. If desired, the image forming apparatus can provide a monochromatic rather than a panchromatic image on the transfer medium.
Since the three different types of microcapsules (i.e. those having red, green or blue dye precursors) are exposed simultaneously, the photosensitive medium need be exposed only once to light representative of the multicolored image. Therefore, the repeated exposure, transfer and fixing of the image required for electrophotographic type copiers and printers is unnecessary. Paper production costs associated with image forming apparatus are considerably lower as compared to copiers or printers using electrophotographic or silver halide photography. Development of the image on the transfer medium is effected using a dry process rather than a wet process as required by silver halide photography. Unfortunately, commercially available image forming apparatus also have several drawbacks.
More specifically, the optical density of the latent image formed on the photosensitive medium is quite sensitive to changes in the level of intensity of light striking the photosensitive medium. Consequently, only a narrow dynamic range of exposure energy levels is available for use. In particular, the range of exposure energy levels for color development (i.e. hardening or softening) of the microcapsules at intermediate tones is relatively small. Therefore, when the photosensitive medium is exposed to a wide range of light energy levels representative of the image to be reproduced, the gradual passing from one tint or shade to another (i.e. "gradation") of the colored image cannot be accurately reproduced.
Conventional image forming apparatus are also unable to reproduce a colored image with hues of high fidelity. The inability to reproduce colored images of high fidelity stems from the maximum and minimum levels of light energy used for forming the latent image on the photosensitive medium being dependent upon the wavelengths of light used.
When exposing the photosensitive medium to light only light representative of the image is used. The photosensitive medium is not exposed to light outside its image forming region. When the photosensitive medium and transfer medium are pressed against one another by the pair of pressure developing rollers, the microcapsules outside the image forming region, that is, in the unexposed region of the photosensitive medium are crushed causing unnecessary and undesirable color development on the transfer medium. Therefore, an overall degraded appearance in the image on the transfer medium is created.
The liquid component flowing from the crushed microcapsules especially in the unexposed region of the photosensitive medium causes the transfer medium and photosensitive medium to adhere to each other. Separation of the photosensitive medium from the transfer medium during the subsequent separation process is difficult. In the event that only the photosensitive medium and not the transfer medium is placed between and crushed by the pair of pressure developing rollers, the contents of the unset (i.e. unhardened) microcapsules in the unexposed region of the photosensitive medium will adhere to the rollers and other elements of the apparatus. Performance of the apparatus deteriorates as a consequence thereof.
Adherence of the photosensitive medium to the pair of pressure rollers can be avoided by applying pressure to the photosensitive medium only when the transfer medium is also present between the pair of rollers. The pair of pressure rollers are, at all other times, separated from each other. When this is done, the leading and trailing edge portions of the transfer medium cannot be pressed against the photosensitive medium. Furthermore, since the pressure developing operation is typically carried out by a pair of cylindrical rollers, the boundaries at the leading and trailing edges of the image formed on the transfer medium are blurred resulting in an unattractive and unacceptable finished reproduction.
Accordingly, it is desirable to provide an image forming apparatus using a photographic process which produces an image of high tonal quality. The photographic process should also produce clean, clear color images on a transfer medium, avoid the need to dispose of waste solution, eliminate the generation of offensive odors, reduce operating costs and be housed within a relatively lightweight and small sized apparatus. The image forming apparatus should provide an acceptable gradation of the image with hues of high fidelity, prevent unnecessary color development on the transfer medium, and substantially reduce adherence of the photosensitive medium to the transfer medium. The apparatus should prevent the contents of the unset microcapsules from adhering to the rollers and other elements of the apparatus. It is also desirable to provide an image forming apparatus which avoids reproducing images having blurred leading and trailing edges.